Cleaning gas-swept heating surfaces of steam generators



April 21, 1970 Filed 001;. 15, 1967 D. A. BROADBENT CLEANING GAS-SWEPT HEATING SURFACES OF STEAM GENERATORS 2 Sheets-Sheet 1 Fig. .l

INVENTOR.

DYSON A. BROADBENT BY 9.. a I

April 1970 D. A. BROADBENT CLEANING GAS-SWEPT HEATING SURFACES OF STEAM GENERATORS F Filed Oct. 15. 1967 a 2 Sheets-Sheet 2 INVENTOR.

DYSON A. BROADBENT BYQ) a 2: 0 -041 United States Patent CLEANING GAS-SWEPT HEATING SURFACES OF STEAM GENERATORS Dyson A. Broadbent, 212 Chapman Ave., Lansdowne, Pa. 19050 Filed Oct. 13, 1967, Ser. No. 675,186 Int. Cl. F22b 37/48 US. Cl. 122-379 12 Claims ABSTRACT OF THE DISCLOSURE Methods and apparatus for cleaning combustion deposits from gas-swept heat exchange surfaces in modern high-temperature steam generators shortly after shutdown. A liquid stream emerging from a nozzle at a velocity of at least 900 feet per second is progressively directed at different portions of the heat exchange surfaces from a distance of five feet or less and preferably in the direction of the flow of combustion gases to wash the surfaces free of deposits. A compact, simply constructed apparatus having hydraulically balanced, opposed, inline nozzles in a head rotated at controlled speed by an air motor and reduction gear speed drive is supported and moved in spaced relationship to the surfaces to be cleaned with liquid cleaning medium exiting the nozzle.

BACKGROUND OF THE INVENTION This invention relates to the cleaning of heat exchange surfaces and more particularly to method and apparatus for cleaning the exterior of gas-swept heat exchange surfaces of water-tube steam generators within a short time after the generators have been shut down.

In modern steam generator apparatus of the type now commonly used in utility and industrial operations where the gaseous combustion products of ash-containing fuel such as coal and oil sweep the exterior surfaces of heat exchange elements, these surfaces become coated with insulating deposits comprising a mixture of various residues of inorganic matter present in the fuel. These hard and tightly adherent deposits reduce the efiiciency of heat ex change and are not removed readily by conventional methods such as by use of cascading shot or by use of soot blowers. Heretofore, no method has been successful in cleaning completely to the bare metal the surfaces of such heat exchange elements as superheaters, reheaters, and economizers.

Concomitantly with the increased use of high-temperature steam generators operating at 1,000 F., and above the formation of deposits on heat exchange surfaces has become more of a proble, especially since these deposits are sometimes very hard and dense, physically resembling concrete.

Chipping, the best of the prior art methods, requires a long period of cooling after shut down of the steam generator because it uses hand labor with men working inside the housing of the steam generator in cramped quarters, using bars, rods, and other hand tools to physically remove the deposits. Such cleaning is not only tedious and time consuming, but is generally incomplete because of the nature of the deposits and because some portions of superheater, reheater, and economizer heat exchange surfaces are inaccessible as a consequence of limitation of the space available for access and manipulation of tools.

Attempts have been made to remove the deposits using streams of water directed through the soot blowers and from externally located fire hoses. Soaking by these methods will ultimately loosen the deposits, but the long time involved makes such methods uneconomical.

I have now developed a method and apparatus for quick 3,507,257 Patented Apr. 21, 1970 and economical cleaning of deposits from heat exchange surfaces a short time after shut down, cleaning to the bare metal in a relatively short time without hand chipping or other physical work within the housing. It now becomes feasible to schedule shut down and cleaning for a week end or other short time when electrical demand is low and the cost of the shut down is lowest.

SUMMARY OF THE INVENTION It is thus an object of this invention to provide a mechanically simple and inexpensive apparatus and an economical, quick method for the cleaning of heat exchange surfaces of the type described.

In the operation of my invention, I direct a superhigh-velocity stream of a liquid cleaning medium, preferably in the direction of flow of the combustion gases, to impinge on and remove the deposit from the heat exchange surface. By super-high-velocity I mean a flow rate of liquid which is but slightly below that which is equal to the velocity of sound in air. I include a velocity which is just subsonic as an upper limit and a velocity of 900 feet per second as a lower limit in my definition of super-high-velocity.

I have found that when a stream. of liquid cleaning medium is directed at a surface from a distance of about five feet or less at super-high-velocity the resultant splashing and spattering upon impact with the surface permits penetration of cleaning medium to those hitherto inaccessible parts of the heat exchange elements not directly in a line of sight with the cleaning device while also atomizing a significant portion of the liquid. This atomization promotes evaporation from the large surface area of the fine droplets and mist so generated, quickly saturating the air enclosed within the steam generator housing and pre-soaking those portions of the deposit not in the immediate path of the main stream while inhibiting evaporation from previously wctted surfaces. The deposit, once thoroughly saturated and softened, is quickly removed from the heat exchange surface by the flowing stream of superhigh-velocity liquid cleaning medium.

For the most effective practice of the method of my invention I use an apparatus which is both compact and easily assembled from commercially available components. The apparatus comprises a tube through which the liquid cleaning medium flows, means for supporting, guiding, and moving the tube forward and backward along a path aligned parallel to the exterior faces of the heat exchange elements to be cleaned, a rotatable head comprising a pair of hydraulically blanced opposed, in-line nozzles and an axial member, the axial member coupled through a rotary joint to the tube, the nozzles aligned to project streams of liquid cleaning medium in a plane which is perpendicular to the axis of the tube, and, means utilizing an air motor for rotating the head at a controlled slow speed independent of the flow rate of the liquid cleaning medium.

Since the apparatus of this invention is readily made,

small enough to enter spaces through which a man of ordinary stature cannot pass, it now becomes possible and practical to clean heat exchange elements heretofore not reached by manual cleaning tools. It also allows cleaning to start soon after shut down before the steam generator has cooled sufficiently to permit men to work inside the housing.

Since the method and apparatus of this invention permits efficient, thorough, and quick cleaning of heat exchange surfaces found in utility steam generators and the like, it is now possible to schedule cleaning of such equipment based upon a short outage during periods of low system load. The outage of a steam generator unit usually means that the turbogenerator is also out of service, as most modern steam generators and turbogenerators are installed as a unit. Because of the high cost of such units, on-site spare units are generally not available and other system generating capacity must be used.

The above and other objectives and advantages of my invention will become more apparent from the following description and drawings in which I have illustrated and described the operation of a preferred embodiment of the invention.

BRIEF DESCRIPTION ORTHE DRAWING FIGURE 1 is a schematic sectional side view of a utility steam generator of the type used in electrical power generation stations.

FIGURE 2 is a side view of a preferred embodiment of the apparatus of this invention in an operating location in the steam generator of FIGURE 1 taken on line AA in FIGURE 1, with the scale enlarged for clarity.

FIGURE 3 is a perspective view of the apparatus of FIGURE 2 and serves to clarify some of the details shown therein.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGURE 1 schematically illustrates the pertinent parts of a large water-tube steam generator of the general type used for electrical power generation and for which the method and apparatus of my invention is specially useful. Such a steam generator produces on the order of one mil lion pounds of steam each hour burning either fuel oil or ground bituminous coal. In the drawing reference symbol represents a housing enclosing and defining the path for the gaseous combustion products through the various parts of the steam generator. Combustion takes place in the furnace primarily in the volume designated by 12 in the vicinity of burners 14. It is to be understood that the interior walls of housing 10' in and adjacent the combustion zone are substantially completely covered with heat exchange surface comprising water tubes as schematically indicated in part by the vertical lining at 16 and 18. Heat exchange elements not in the combustion zone include low-temperature superheater 20, high-temperature superheater 22, reheater 24, and economizer 26. These heat exchange units are suitably interconnected with each other, with steam drum 28, and with a steam turbine, not shown, as is known in the art.

Combustion gases formed in volume 12 are partially cooled by radiant heat exchange with heat exchange surfaces 16 and 18 prior to sweeping the surfaces of the heat exchange elements comprising high-temperature superheater 22, reheater 24, low-temperature superheater 20, and economizer 26, in turn, transferring heat thereto by convection. Because of the cooling and change in direction of the combustion gases in this region the entrained mixture of residues which is initially at a temperature above its melting point solidifies and forms the most tenacious deposits on the elements of high-temperature superheater 22, reheater 24, and low-temperature superheater 20.

In the operation of my invention during the initial washing I have found that I can get best results by positioning the washing device to clean the surfaces of reheater 24 from the direction in which the combustion gases normally flow, the washing device being located just to the left of line AA in FIGURE 1.

Referring now to FIGURE 2, taken on line AA of FIGURE 1 with an enlarged scale, tube 30 is a conduit for liquid cleaning medium shown joined to the non-rotational side of rotary joint 32. The rotational side of joint 32 is joined to a rotary head made up of axial tube 34, pipe T 36, and a pair of radial in-line opposed nozzles 38. Reversible, controllable speed air motor 40, mounted on the housing of gearbox 42, is coupled to drive a reduction gear assembly 44, gearbox 42 and reduction gear assembly 44 being fastened together to form a rigid unit with axial tube 34 passing through a suitable opening in gearbox 42 and coaxially through the hollow output drive shaft of reduction gear assembly 44. As is understood by those skilled in the art the outer diameter of axial tube 34 is sized for slip fit with the aforesaid hollow output drive shaft and caused to rotate with the drive shaft by fastening with a key. The speed of air motor 40 is adjustable by control of the pressure of its air supply and is adjusted to drive the rotary head at a rotational speed of between about 7 and 10 revolutions per minute.

A rigid frame, comprising generally vertical frame members 46 and 52, and generally horizontal frame members 48 and 50, fastened to reduction gear assembly 44 and the housing of gearbox 42, is provided with removable shackles 54 for suspension of the apparatus from cable 56 which is stretched taut, passing through openings in the housing, not shown, and is fastened externally to provide a support and guide for movement of the washing device. Pull lines 58 and S8, fastened to opposite ends of frame member 50 near shackles 54, are used to position the washing device, as will be explained more fully hereafter. Cable 56 is located so that the washing device is positioned within from 3 to about 7 feet, and preferably at about 5 feet from the closest surface of reheater 24 and can be moved parallel to the exposed face of reheater 24.

The non-rotational side of rotating joint 32 is restrained, as by clamp assembly 60, holding rotating joint 32 in fixed relationship with frame extension 61 to maintain the alignment of axial tube 34 and support the overhung weight of high-pressure hose 62 which supplies liquid cleaning medium to the washing device through tube 30 and is connected therewith at pipe elbow 64. Liquid cleaning medium is supplied through high-pressure hose 62 by a suitable high-pressure pump and flow control ap paratus, not shown, at ground level. Air hose 66 supplies air at controlled pressure to air motor 40 from an air compressor, not shown.

FIGURE 3, in which like reference symbols refer to like elements of the apparatus of this invention, gives a somewhat clearer view of the location of the elements comprising the rotary head, and shows, by means of the arrows associated with 58 and 58' the direction of movement of the washing device with respect to reheater 24. The curved arrows indicate the direction of rotation of nozzles 38.

Nozzles 38, machined from solid stock, are threadably joined into the opposed ends of T 36. Interchangeable matched pairs of nozzles are provided for flow of liquid cleaning medium by drilling identical axial bores of predetermined diameter in each member of a matched pair of nozzles. Opposed, in-line, identical nozzles are used for hydraulic balance and simplicity of design, eliminating need for bulky and complicated thrust bearings and the like. With the hydraulically balanced nozzles, aligned as shown, there are substantially no radial or axial reaction forces acting on the rotating head and a simple, compact rotating joint can be used.

When fully assembled, a preferred embodiment of the washing device of this invention is about one foot long and six-inches in diameter. These dimensions are exclusive of the frame, air hose, and high-pressure hose.

In a demonstration of the utility of my invention using this apparatus and following a preferred mode of operation, a 1,125,000 lb./hr. steam generator of the type shown in FIGURE 1 was cleaned in 22 hours with a crew of four men. The heat exchange surfaces were all free of all deposits to bare metal at the completion of the cleaning. An earlier manual cleaning of the same s team generator during an overhaul required a crew of 10 men working one shift a day for 21 days. At the completion of the manual cleaning, hard tenacious deposit still remained on portions of the low-temperatuE superheater and reheater heat exchange surfaces.

A more complete and better understanding of the practice of my invention can be had by reference to the following example.

Example A steam generator of the type described was shut down and cooling started by use of'the normal air supply fans. Five hours after shut down one-half inch diameter steel cables were fished through and stretched across the width of the steam generator at preselected locations adjacent the front face of the reheater, suitable openings having been made previously. in the housing Where inspection hatches were not available. Similarly, cables were strung at other locations. The washing device of this invention was then supported from the cable by its shakles and the pull lines led out through the opposite openings.

The washing device used in this operation was assembled from one-half inch pipe size heavy duty tube and fittings. The rotary head was driven by a commercially available air motor capable of developing /3 horsepower at' a speed of 350 r.p.m. with 100 p.s.i.g. air supply pressure. Suitable gearing was provided to couple the air motor to a double reduction helical gear speed reducer commercially avialable from Boston Gear works, Quincy, Mass.

The initial operation, starting at the front face of the reheater, was a period of soaking to wet and saturate the deposit. For this step the washing device was provided with matched nozzles having a bore diameter of 0.156 inch. Seventy-five gallons per minute of water at ambient temperature was used as the liquid cleaning medium. The water was supplied at a pressure of 5000 p.s.i.g. The calculated nozzle exit velocity for these conditions is 630 ft./sec.

With the washing device positioned so as to move in a path about 5 feet from and generally parallel to the face of the reheater heat exchange surfaces in the space between the high-temperature superheater and the reheater, and with the head rotation adjusted to about 7 rpm. using 18 p.s.i.g. air pressure to the air motor, the washing device was advanced at a rate of between 3 to 4 feet per hour by means of the pull lines. As the soaking period progressed the washing device was relocated to other cable locations to assure complete coverage with water and complete soaking of all heat exchange surfaces. During this operation the interior of the steam generator was observed to be filled with a fine mist and spray, and even those surfaces most distant from the points of impact of the streams from the nozzle were well wetted.

At each cable location, when the washing device had reached the opposite end of its axial traverse the direction of rotation of the nozzles was reversed and the washing device return to its initial position at the same rate of advance as used in the forward movement.

Upon completion of the soaking operation, the nozzles were changed to a matched pair having a bore of 0.125 inch diameter. Water was pumped to the nozzles at a pressure of 7,000 p.s.i.g. and at a flow rate of 75 g.p.m. The calculated nozzle exit velocity for these conditions is 980 ft./sec.

Using the previous cable locations and the same nozzle rotational speed as in the soaking operation, again starting at the reheater, the washing device was then advanced at a rate of between about to 20 feet per hour, and moved forward and backward along the cable with corresponding changes in the direction of nozzle rotation until the heat exchange surface was seen to be clean. The washing device was then relocated to repeat the super-high-velocity wash at a new location.

During both the soaking and the super-high-velocity washing operations the run-off water was allowed to drain through existing ash piping to a settling basin.

The relatively low volume of water used in this operation permitted existing facilities to handle the run-off.

In the cleaning process described in the above example I used a water flow rate of about 600 ft./sec. at the nozzles until all of the deposit was thoroughly soaked. I used this lower velocity because my experience with the presently available commercial rotary joints indicate that the useful service life of these joints can be prolonged if I minimize the operating period of use at nozzle velocities in excess of about 900 feet per second. I believe the shortened service life of the rotary joint when operated with liquid flow above 900 feet per second is associated with the extremely high pressure needed to produce these super-high velocities. In my judgement, mechanical redesign of the rotary joint will reduce wear at these extremely high pressures and permit long periods of uninterrupted operation at about 7,000 p.s.i.g. For the present, however, for reasons of economy, I prefer to use a nozzle velocity of about 600 feet per second initially until all deposits have been soaked. Such soaking is useful but is not critical in the practice of my invention, since I find that the time required to clean a bare metal using super-high-velocity washing is significantly reduced if the deposit has first been soaked thoroughly.

While in accordance with the provisions of the statutes I have illustrated and described herein the best form and mode of operation of the inventiton now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed herein without departing from the spirit of the invention. In this regard, it is readily apparent that pulleys can be provided instead of shackles to perform the same function, and that steam generators can be provided with preinstalled support structures of rails and the like obviating the need to string cables and further shortening the waiting period after shut down before cleaning can start. Similarly, a pulley system and motor drive device can be used to advance the washing device at a uniform rate. Liquid washing media other than water can be used; Wetting agents can be added to the liquids used; water can be heated to further promote wetting; and, the water or other liquid can be collected and recirculated.

As presently practiced, the advantages of the invention reside in the simplicity of the method and apparatus which allows cleaning of many varied types of heat exchange surface with one basic device. The apparatus is readily assembled from existing, commercially available components. When used with steam generators, cleaning can be started without the normal long waiting time for cooling. In this regard, I have found it advantageous to start the cleaning operation as soon as possible after shut down, since the heat retained within the steam generator seems to assist in the rapid and complete removal of the deposits.

It will also be obvious to those skilled in the art that certain features of my invention may sometimes be used to advantage without a corresponding use of the other features and that numerous modifications or alterations thereof may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims.

I claim:

1. The method of cleaning gas swept heat exchange surfaces associated with water-tube steam generators and the like, comprising:

after shut-down directing a stream of super-high-velocity liquid cleaning medium at the said surfaces.

2. The method of claim 1 wherein the said stream is directed at the said surfaces from a distance of about five feet and less.

3. The method of claim 1 wherein the said liquid cleaning medium is water.

4. The method of claim 1 wherein the said stream has additional velocity components mutually perpendicular to each other and to the direction of flow of the said stream.

'5. The method of cleaning adherent deposits from the surface of heat exchange equipment of the type described during shut-down, comprising:

providing super-high-velocity streams of liquid cleaning medium at hydraulically-balanced nozzles; and

rotating said nozzles around an axis perpendicular to the said streams while advancing the said nozzles along the said axis, said axis parallel to, spaced apart from, and exterior to the face of said heat exchange equipment.

6. The method of claim 5 wherein the separation between the said axis and the said face is no greater than about five feet.

7. The method of cleaning deposits from the heat exchange surfaces of a utility steam generator, comprising the steps:

-( a) shutting down the said steam generator;

'( b) providing line-of-sight openings in the opposite walls of the housing of the said steam generator;

() stringing a support cable through the said openings with said cable substantially parallel to a selected one of the said surfaces;

((1) suspending a washing device from the said cable;

(e) providing a flow of liquid cleaning medium to the said Washing device under conditions to produce a super-high-velocity stream at the exit of an outwardly directed nozzle of said washing device;

(f) advancing the said washing device in a direction parallel to the length of said cable while rotating the said nozzle around an axis parallel to the said direction;

(g) repeating step (if) in opposite directions as many times as required to clean the said selected one of the said surfaces; and

(h) repeating the sequence of the steps from (b) through (g) in as many locations as are required to clean all the said surfaces.

8. In the method of cleaning deposits from the gasswept heat exchange surfaces of a steam generator using super-high-velocity streams of liquid cleaning medium, the step comprising directing the said streams at the face of a heat exchange unit from a distance of about 5 feet, said 4 streams directed to follow the path of the combustion gases through the said steam generator, said steam generator being shut-down.

9. In combination, a furnace in which combustible material is burned, heat exchange surface means are assosociated with said furnace upon which solid matter resulting from the combustion adheres, heat exchange surface cleaning means associated with said heat exchange surface means for periodically removing deposits of said solid matter from said heat exchange surface means, said cleaning means comprising a tube through which a liquid cleaning medium flows, means for guiding and moving the said tube forward and backward in a direciton parallel to the axis of said tube and in spaced apart relation with the exposed face of said heat exchange surface means, head means rotatably coupled to said tube, said a head means including a radial nozzle, means comprising and air motor and reducing gears for rotating said head means on an axis parallel to the said direction, and means for producing a super-high-velocity stream of said liquid medium exiting the said nozzle.

10. A washing device of the type described, employing a stream of liquid cleaning medium at super-high-velocity, comprising:

rotatable head means for providing said stream at said velocity; and, means for rotating said head independent of the flow rate of said stream. 11. The washing device of claim 10 in which said rotatable head means includes hydraulically balanced nozzles.

12. The washing device of claim 11 in which said means for rotating said head comprises an air motor.

References Cited UNITED STATES PATENTS OTHER REFERENCES Diamond Valve-in-Head Soot Blowers, bulletin 137, Diamond Power Specialty Corp, 1921, p. 15.

KENNETH W. SPRAGUE, Primary Examiner US. Cl. X.R. 122-392 

